Typical standard fusebox doesn't have a fuse holder in the very end accessory slot. When the tach was added that wiring included the additional fuse clip. Tach has 2 contact posts. l
Longer one gets the wire from this fuse. Shorter post has a wire to the neg' side of coil. This wire in a separate grommet through the firewall. If all this looks correct and the tach works as it should, probably not related to the fault.
If a tach and wiring was added... it could be the culprit. Still seeking clues.